| It is of great significance to strengthen the basic research on the ductile fracture of natural gas pipeline in order to avoid the fracture failure for the protection of country’s energy security.The dynamic crack propagation of natural gas pipeline involves the coupling effect of fluid,structure and fracture,which is a complex process with multiple mechanical behaviors interwoven.Relevant studies have shown that the traditional two-curve model based on Charpy impact toughness is no longer applicable for high-grade pipeline due to its high strength and toughness.Thus,it is urgent to establish a new crack arrest control method based on a more appropriate fracture parameter.Crack Tip Opening Angle(CTOA),with the advantages of clear definition and convenient use,is one of the most promising crack arrest toughness parameters in the field of crack arrest control for natural gas pipeline.However,it is far from mature in terms of testing,characterization and application.In view of that,this paper aims to establish a crack arrest prediction model suitable for high-grade pipeline taking X80 pipeline as the research object and critical CTOA as the fracture parameter.The main research work of this paper is as follows:(1)In view of the problem that it is difficult to obtain CTOA accurately during the test,the basic assumptions of Martinelli-Venzi plastic hinge model for three-point bending specimen are introduced in this paper.Based on the fact that the load decreases near-linearly with the increase of displacement during crack steady-state propagation,an analytical model is established to determine critical CTOA directly from the load-displacement curve.The difficulties in CTOA testing caused by the unaccurate determination of the crack tip and fracture surface can be avoided by that analytical model.The transient effects at the beginning and end of crack propagation can also be eliminated.The reliability and repeatability of the test results can be improved significantly and the artificial error can be reduced.(2)The test results of CTOA for laboratory small-size specimen are difficult to be directly used for the full-scale pipeline due to their different constraint states.In view of that,a novel constraint parameter based on the area surrounded by the contoure of quivalent plastic strain at the crack tip is proposed.With the help of the quantitative description about the relationship between the constraint parameter and critical CTOA and the in-depth analysis about the effects of various related factors,a correlation model between the two is established and verified by experiments.Based on the correlation model,the mutual transplantation of the crack arrest toughness between laboratory specimens and pipelines can be initially realized,so that it is possible to directly use the fracture parameter obtained from the laboratory specimen for pipeline design.(3)The traditional numerical simulation method cannot adjust the internal pressure distribution of pipeline in real time with the crack propagation,which leads to the inaccurate simulation results.In view of that,a basic assumption that the crack tip position changes near-linearly with loading is put forward based on the full-scale pipe burst test data.Based on that,an iterative loading method incorporating real-time prediction of the crack tip position is proposed with the gas decompression model.The dynamic crack propagation of pipeline is simulated based on the damage mechanic method and the reliability of the loading method is verified by full-scale burst tests.The new numerical simulation method for dynamic crack propagation of natural gas pipeline formed in this paper can overcome the shortcomings of traditional methods,and lay a solid foundation for the subsequent research of crack arrest behavior of natural gas pipeline.(4)At present,the evolution laws and influence factors of CTOA during the crack propagation and arrest of natural gas pipeline are still not clear.In view of that,the dynamic crack propagation behaviors of natural gas pipeline,including the crack velocity,crack arrest length,the thinning ratio of pipe wall thickness,crack front morphology and residual plastic deformation,are systematically studied based on the numerical simulation technology for dynamic crack propagation of pipeline developed in this paper.The effects of design coefficient,geometry size,inertia backfill effect and material property of pipeline steel are all considered.The evolution laws of CTOA are revealed and the effects of the above factors are clarified.On this basis,a prediction model that can directly determine critical CTOA by the key influencing factor of pipeline diameter is establised,which provides the basis for the construction of a CTOA-based pipeline crack arrest control method.(5)In view of the poor applicability of the current crack arrest control method for high-grade pipeline,some work has been done in this paper.Firstly,the relationship between the local fracture dynamic behavior of the pipeline crack tip and the change of macro-energy is analyzed based on the fact that the pipeline crack propagation is controlled by critical CTOA.Then,an analytical model of crack arrest pressure based on critical CTOA is established with the energy balance theory.On this basis,a prediction model of crack velocity based on critical CTOA is established by exploring the relationship between crack front pressure and crack velocity under different crack arrest toughness—critical CTOA.Finally,a new two-curve model(CBTCM)taking critical CTOA as the crack arrest toughness parameter is developed for crack arrest prediction and design of the natural gas pipeline,which provides a new reference for high-grade pipeline crack arrest control. |
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